1. Field of the Invention
The present invention relates to a vertical structure III-V group compound semiconductor light emitting diode (LED) device and a method of manufacturing the same, and more particularly, to a vertical structure semiconductor LED device capable of enabling wafer bonding at a low temperature and preventing occurrence of stress and a method of manufacturing the same.
2. Description of the Related Art
In general, III-V group GaN semiconductors shown as AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, and 0≦x+y≦1) are compound semiconductor materials suitable for emitting light in ranges of blue and ultraviolet and used for light emitting diode (LED) devices of ultraviolet and blue. General GaN LEDs include an n-type GaN semiconductor layer, an active layer, and a p-type GaN semiconductor layer, and two electrodes including an n-side electrode and p-side electrode. Since a sapphire substrate used for a substrate for growth is an insulating material, to apply current to the active layer, a portion of the GaN semiconductor is etched and the n-side electrode is formed on an exposed portion of the n-type semiconductor layer. In the horizontal structure GaN LED device, since all of the two electrodes are disposed above the device, there are disadvantages in aspects of current diffusion and electrostatic.
Instead of the horizontal structure GaN LED devices having such disadvantages, recently, vertical structure GaN LEDs using a conductive SiC substrate for a substrate for growing a GaN semiconductor have been used. However, in this case, it is required to use a high priced SiC substrate. Vertical structure GaN LEDs in another form are manufactured by a process of bonding a conductive substrate (wafer bonding) and a process of separating or removing a sapphire substrate. For example, Korean Patent Publication No. 10-2004-0058479 discloses a vertical structure GaN LED including a conductive substrate such as an Si substrate bonded to a GaN semiconductor layer by a conductive bonding layer.
FIG. 1 is a cross-sectional view illustrating a conventional vertical structure GaN LED device. Referring to FIG. 1, a vertical structure GaN LED device 10 includes an n-side electrode 17, an n-type GaN semiconductor layer 16 formed below the n-side electrode 17, an active layer 15, a p-type GaN semiconductor layer 14, and a reflective metal layer 13. Also, below the reflective metal layer 13, a conductive substrate 11 is bonded by a conductive bonding layer 12 and a p-side bonding electrode 18 is formed at the bottom of the conductive substrate 11. The conductive bonding layer 12 may be formed of a eutectic metal having a low melting point such as AuSn, and the conductive substrate 11 may be formed of an Si substrate having an excellent thermal conductivity. The vertical structure GaN LED device 10 shows more improved light emitting efficiency, heating characteristics, and electrostatic discharge immunity that those of a horizontal structure GaN LED device and has a light emitting area larger than that of the horizontal structure LED device in the same chip area.
FIGS. 2A through 2D are cross-sectional views illustrating a method of manufacturing the vertical structure GaN LED device 10. Referring to FIG. 2A, on a sapphire substrate 21, which is a substrate for growth, GaN semiconductor layers 13, 14, and 15 and a reflective metal layer 13 are formed. Referring to FIG. 2B, a conductive substrate 11 is bonded to a GaN semiconductor layer side (wafer bonding process) by using a conductive bonding layer 12 having a low melting point of 330° C. or less. Referring to FIG. 2C, the sapphire substrate 21 is separated or removed by one method of laser lift off and chemical mechanical planarization. After separating the sapphire substrate 21, referring to FIG. 2D, electrodes 17 and 18 are formed on an n-type semiconductor layer 16 exposed by the separation of the substrate 21 and a conductive substrate 11.
According to the conventional manufacturing process, in a wafer bonding process, substrate bonding is performed under a uniaxial pressure by the medium of eutectic metals having a relatively low melting point of 330° C. or less, such as AuSn, AuIn, PdIn, Sn, and In. Since a high atmosphere temperature of 450° C. or more and a high pressure of 0.4 kgf/cm2 are required when directly bonding a conductive substrate by using a metal having a high melting point, such as Au, the metal having a low melting point is used. In a condition of such the high temperature and high pressure, due to a difference between thermal expansion coefficients of the sapphire substrate and conductive substrate, a great stress is applied to a substrate, thereby bending the substrate or generating a lot of defects and cracks in the GaN semiconductor layer. Particularly, in a laser lift off process after wafer bonding, a bonding structure itself is destroyed, thereby greatly reducing a process yield.
However, in the case of the wafer bonding using an eutectic metal having a low melting point as a bonding material, there is still a stress generated due to a difference between thermal expansion coefficients of a substrate for growth and a conductive substrate at a temperature from 200 to 330° C. Also, a thermal processing temperature to improve ohmic characteristics of the n-side electrode layer formed on the n-type semiconductor layer exposed after removing the substrate for growth is limited to be less than a melting point of the eutectic metal, approximately, 330° C. or more, it is difficult to obtain suitable ohmic characteristics.
The described problems due to a metal bonding material used in wafer bonding of a vertical structure LED manufacturing process may occur not only in a GaN semiconductor LED manufacturing process but also a vertical structure LED device manufacturing process using other III-V group compound semiconductor such as AlGaInP and AlGaAs semiconductors.